An increasing variety of lighting applications utilize electronic type emitters as light sources. Examples of such emitters include solid state light sources, such as light emitting diodes (LEDs) and organic light emitting diodes (OLEDs) as well as plasma type light emitters. An LED produces light output, when a voltage across two terminals thereof (e.g., anode and cathode) exceeds the LED's forward voltage so that forward current can flow through the LED. The intensity of light output from the LED is primarily governed by the amount of forward current flowing through the LED. In order to dim a light emitted from the LED, the forward current flowing through the LED needs to be manipulated. There are two commonly used methods for dimming lights from LEDs. One is Pulse Width Modulation (PWM) Dimming, and the other is Analog Dimming. Both methods result in changing the average current through the LEDs and hence provide a visual appearance of changing intensities of light output from the LEDs. FIG. 1 illustrates an example of an LED's relative luminous flux characteristic, i.e., a relationship between forward current and relative luminous flux. FIG. 2 illustrates an example of an LEDs power consumption by depicting a relationship between forward current and forward voltage.
There is an industry-wide consensus that Analog Dimming may be superior to PWM Dimming. However, Analog Dimming has a drawback of undesirable color variation. In a given LED, if the peak current is varied, the current density (or J) also varies. More particularly, in a Gallium Nitride (GaN) based LED system (for example, Blue and Green type LEDs), a varying current density may lead to not only a varying intensity output but also a varying chromaticity output. In other words, in GaN based materials, Analog Dimming may lead to both intensity and chromaticity variations. While the intensity variation is a desirable effect of dimming, the associated chromaticity variation may not be a desirable one. For example, referring to the graph in FIG. 3, with Analog Dimming in Green LEDs, the chromaticity ((x, y)-coordinates of five connected dots in FIG. 3) shifts due to different forward currents of the LEDs used to produce light (at the five connected dots in FIG. 3). Moreover, as shown in FIG. 4, this shift in chromaticity results in changing dominant wavelength. Hence a need exists for techniques and equipment for color correction of a light emitted from a lighting system to correct for a color change with Analog Dimming of the light.
Furthermore, almost all LEDs show degradations in light output over time during the LEDs' lifetime. FIG. 5 illustrates an example of an LED's lifetime degradation characteristic (i.e., hours used vs. light output). More particularly, FIG. 5 shows that light output of the LED has degraded by 14 percent after ten thousand hours. Hence, when Analog Dimming current density correction is applied, there is still room for further improvement in correcting for changes in color or lumen output of LEDs, either due to temperature changes of the lighting system or due to the LEDs' lifetime degradation.